Instrumentation amplifiers having a DC response all suffer from the appearance of error or unwanted DC signals among their output signals, wherein some, if not all, of these error signals arise from non-linearities or imbalances within the amplifier. These imbalances, even if corrected, are subjected to changing environmental and electrical conditions wherein those corrections are no longer optimum. Typically, in commercially available instrumentation amplifiers which include operational amplifiers (the operational amplifiers having an output and a pair of inputs including an inverting input and a non-inverting input relative to the output), the balance of the input voltages and currents have the most significant effect in contributing to such error signals. The unwanted DC output signals may also arise from an external DC bias present on the amplifier input signals.
As mentioned above, varying operating conditions make static or one-time adjustments of the amplifier circuit inadequate for long term correction. Attempts have been made to stabilize the operating characteristics, such as incorporating thermal ovens to provide a uniform and constant thermal environment, and highly accurate voltage regulators to stabilize the necessary power supply voltages and currents. Even when such measures are implemented, there will always remain a significant error component within the output signal. One approach shown in prior art techniques is to remove the input signal to the operational amplifier and to measure the output signal of the operational amplifier relative to a threshold or reference point, typically the ground or common terminal of the amplifier system. The difference signal is then fed back and added to at least one of the operational amplifier inputs to reduce this difference signal. In this way the operational amplifier error output signal is adjusted to zero volts. The operational amplifier input signal is reapplied and the correction signal is maintained at the previously selected level by analog or digital storage means. These storage means may include a large storage capacitor either used independently or in an integrator-type circuit or may be a digital circuit comprising a register providing a correction signal represented by the binary value of the register which, when converted by a digital-to-analog converter to an analog signal, provides the appropriate correction voltage to the operational amplifier input. This technique is limited by the resolution of the comparison means to measure the output signal from the operational amplifier relative to the reference point, and the storage means to provide a definable correction voltage which provides a suitably accurate correction signal to the operational amplifier input.